Contribution of Roots and Shoots of Three Summer Cover Crops to Soil C and N Cycling Post-Termination

Although summer cover crops (CCs) have relatively short growing periods, they can significantly enhance soil health by contributing to carbon (C) and nitrogen (N) cycling. Three summer CCs—including oat, buckwheat, and pea—were planted in June–July and evaluated for their biomass, allocation of assimilates to roots, C and N yield, and residue decomposition patterns after termination in a 14-week period. Total biomass (roots + shoots) was highest in buckwheat (5822 kg ha−1), followed by oat (4836 kg ha−1) and then pea (20 22 kg ha−1). Across species, the allocation of assimilates to roots decreased from 34% at 30 days after planting to 18% at termination. Total C yield was 2409, 1941, and 808 kg ha−1 for buckwheat, oat, and pea, respectively, with root C content considerably lower than shoot C content. The initial carbon-to-nitrogen (C:N) ratios in the roots and shoots of pea were substantially lowest among the species and remained below the 25:1 threshold, indicating potential for net N mineralization. In contrast, oat and buckwheat exhibited initial C:N of 40–50 in roots and around 30 in shoots. These ratios shifted during decomposition. After a 14-week decomposition period, all CCs had released over 50% of their root and shoot biomass. However, the release of their C and N did not directly align with biomass decay. Approximately 70% of the C in roots and shoots of oats and buckwheat remained unreleased after 14 weeks. The slow N release from oat and buckwheat residues suggests potential N immobilization, which could lead to nitrogen deficiency in subsequent crops.